As is known, splitting a thin film may be achieved by implantation of chemical species in a source substrate, for example of silica, to induce the formation of a zone of defects at a particular depth. These defects may be micro-bubbles and/or platelets and/or micro-cavities and/or dislocation loops and/or other crystalline defects, locally disrupting the crystalline quality of the material; their nature, density and size are strongly dependent on the species implanted (typically hydrogen) as well as on the nature of the source substrate. A heat-treatment may then be applied to enable the development of specific defects present in the weakened zone, which enables splitting of the thin film from the source substrate to be achieved later. This has in particular been described in U.S. Pat. No. 5,374,564 and developments thereof, such as described in U.S. Pat. No. 6,020,252.
However, spontaneous splitting during thermal annealing is sometimes ill-suited to certain situations, for example when the substrates brought into contact have different coefficients of thermal expansion. Moreover, it is known in the art (see for example US published patent application no.—2003/0134489) that, in the case of a fracture obtained by thermal means, and under certain conditions, the latter preferentially commences at a localized place on the wafer, which can sometimes lead to surface non-homogeneities that are reflected in the form of a “marbled” type appearance. These irregularities also appear in the case of a purely thermal fracture when attempting to overfragilize by implanting an overdose of a species such as hydrogen in such a manner as to facilitate the fracture thereof or to minimize the thermal budget (temperature-duration) subsequently applied to the wafers.
When the splitting is produced at high temperature (typically near approximately 500° C.), among the technological problems sometimes encountered, mention should be made of the roughness of the surface as well as the degradation of the film transferred during the thermal splitting. This renders the following treatment steps more difficult (for example: the transferred film must be polished more, there is a risk of crystalline defects being created during the following treatments, etc.). Furthermore, in heterostructures (comprising a superposition of substrates of different materials), another technological problem encountered is the presence of a field of very high stresses in the various films in contact, during the heat treatment, due to the difference in the coefficients of thermal expansivity of the various materials placed in contact. This may induce the degradation of the heterostructures if the thermal splitting occurs at a temperature higher than a critical temperature. This degradation may, typically, be the breakage of one or both substrates brought into contact and/or be the unbending of the substrates at the bonding interface.
This is the reason why it may be desired to achieve the splitting at lower temperature.
One way to achieve splitting at low temperature is to “play” with the implantation conditions. For example, an excess dose of the implanted species may increase the weakening of the implanted zone and cause splitting at low temperature by providing an external force.
Splitting may also be effected by applying an external force that causes the fracture in the weakened area until the thin layer is detached, generally after heat treatment. See in particular U.S. Pat. No. 6,225,192.
It is important to note that, for a given substrate and given implantation conditions, it is not only the treatment temperature that conditions the subsequent conditions of splitting of the thin layer, but also the duration of that treatment, which is reflected in the thermal budget concept (see French Patent No. FR-2 767 416); as for the provision of mechanical energy, it is applied for example by a “guillotine” type tool (see PCT Published Patent Application No. WO 02/083387).
Thus Henttinen et al. (2000) showed that, if the source substrate is a wafer of silicon, a dose of hydrogen ions implanted at 1×1017 H+/cm2 (i.e. 5×1016 H2/cm2), enables splitting by a mechanical force after performing the following steps: treatment, as for the target substrate, by a plasma chemical activation; cleaning of RCA1 type, bonding at ambient temperature of the source substrate onto the target substrate, and annealing at 200° C. for 2 hours. (K. Henttinen et al., Applied Physics Letters, Volume 16, Number 17, 24 Apr. 2000; pp. 2370-2372). The mechanical force utilized came from a blade inserted at the bonded interface to initiate the splitting.
This approach, although reducing the roughness of the transferred surface (by of the order of half with respect to conventional splitting solutions that are purely thermal and without plasma activation), involves slow and jerky propagation of the fracture wave. Thus Henttinen reports that each forward movement of the blade leads to the propagation of a fracture that is stabilized over a certain distance after two minutes.
This type of mechanical splitting therefore consists of introducing a blade from the edges of the structure and moving this blade forward over virtually all of the bonded structure, as if to “cut it out” along the weakened zone; the term ‘assisted splitting’ is sometimes used, since the role of the tool (such as a blade) is to propagate the fracture wave from one edge of the structure to the other.
This type of fracture leads to the following defects, at the future surface freed by the splitting of the thin film: 1) crown defect (non-transferred zone, at the periphery of the final product), for example related to a local bonding energy too low with respect to the rest of the interface, and to the introduction of tools to start off the transfer, 2) lack of uniformity (low frequency roughness) of the thickness of the thin film transferred, in particular due to the fracture wave assisted mechanically, thus irregular, by fits and starts, which then necessitates treatments, such as polishing, which it is however generally sought to avoid, and 3) difficult industrial implementation, given the use of a tool which accompanies the propagation of the fracture, which implies an individual treatment of each structure (or wafer).
Moreover, it has been found that, if the thermal budget is too low, the transfer of the thin film is of poor quality, whereas if it is too high, fracture of one of the substrates may occur in the case of a heterostructure. It is therefore clear that in theory there is a narrow window for the operating parameters (of course linked to the conditions, in particular the doses implanted, the nature of materials, the annealing temperatures, and the like); now, this narrowness constitutes a heavy constraint for industrial exploitation.
Most of these drawbacks are found in the case of the splitting of a thin film in a homogeneous substrate (with a single component material (silicon-on-insulator (SOI), for example).
The splitting of the thin film is of course also determined by the choice of the chemical species implanted.
It was indicated above that hydrogen is generally implanted, but other options have been proposed, in particular by implanting helium.
Combination may even be made of two different chemical species.
Thus Agarwal et al. (1998) found that the fact of implanting both hydrogen and helium enabled the total implanted dose of ions to be reduced, apparently due to the different roles played by hydrogen and helium: the hydrogen interacts with the Si—Si bonds broken by the implantation, to create Si—H bonds, resulting in a high density of platelet type defects of a size of the order of 3-10 nm (termed H-defects of platelet type), whereas helium, which does not act chemically, leads to the appearance of a lower density of larger defects (size greater than 300 nm approximately). (A. Argawal et al. Applied Physics Letters, volume 72, Number 9, 2 Mar. 1998; pp. 1086-1088). The heat treatments envisaged in that article are 450° C. for 20 min or 750° C. for 20 s, which necessarily implies the drawbacks mentioned above in relation to splitting at high temperature.
This hydrogen-helium combination has also been studied, in a more theoretical manner, by Cerofolini et al. (2000) [3], who noted that pressurization of the defects was greater with the implantation of helium than with that of hydrogen, and that the heat treatment could have different effects according to the temperature chosen: annealing between 150° C.-250° C. leads to a reduction in the number of Si—H bonds, annealing in the range 300° C.-450° C. leads on the contrary to an increase in that number, whereas annealing above 550° C. tends instead to reduce that number again. (G. F. Cerofolini et al., Materials Science and Engineering B71-2000, pp. 196-202). However that article does not deduce practical conclusions therefrom as to the manner of obtaining thin films of good quality (in particular in relation to the state of the surface) for a moderate cost.
An object of the invention is to alleviate the drawbacks described above.
More particularly, the invention relates to a method of transferring a thin film that can be carried out at low temperature (in order to limit the high mechanical stresses when using materials having large coefficient of expansion differences), that can be effected collectively and limiting the defects cited above during splitting of the thin layer, in particular by preventing jerking of the fracture wave. In other words, an object of the invention is to obtain, for a moderate cost, thin films of high quality, thereby avoiding at the same time the drawbacks of a heat treatment at high temperature and those related to the utilization of a tool for assisted splitting, and those related to an additional treatment for reducing roughness after splitting.